The present invention relates to a system and method for re-establishing the earth-pointing attitude of a spacecraft from an arbitrary non-earth-oriented attitude.
For any spacecraft, it is desired to maintain a certain orientation of the spacecraft with respect to various objects. For geosynchronous spacecraft, the normal mission attitude is such that the spacecraft points its communications antennas at some location on the Earth""s surface. Typically, the Earth or one or more stars provide reference vectors that permit the orientation of antennas, sensors, and other devices that make communications with the spacecraft possible and are necessary to carry out mission operations. Despite the presence of sophisticated spacecraft on-board fault detection and correction systems, anomalies can occur that cause a loss of earth pointing control.
During non-earth-pointing periods the spacecraft cannot perform its mission, and this may cause a loss of revenue or other negative impacts to companies that rely on the spacecraft to support their businesses. Therefore, to limit these negative impacts, it is desired to have a spacecraft that can re-establish normal earth pointing as quickly as possible, and with minimum interaction with ground operators. Unfortunately, current re-acquisition methods do not provide the called-for quick recovery capability.
According to one example, if the inertial reference of some spacecraft is corrupted, the satellite must execute a full sky search to locate the Earth. In the worst-case scenario, the Earth search may involve up to four 360 degree rotations about the spacecraft roll axis and three large-angle pitch/yaw attitude slews. The entire re-acquisition sequence can take up to 5 hours, during which time the spacecraft may be in a poor power, thermal, and communications attitude. Even after Earth-lock is established, another 19 hours may be required to restore normal pointing performance. This stems at least from the fact that sun sensor yaw data may only be available for a limited portion of the orbit, e.g., for approximately 5 hours or less.
With the trend to larger spacecraft, the situation is even worse, because lower search rates must be used to accommodate limitations imposed by the available thruster torque capability or reaction wheel momentum storage capability. Virtually no spacecraft is immune to re-acquisition performance issues. According to one example utilized in a large spacecraft, to re-acquire the normal mission attitude, the spacecraft first locates and locks on the Sun, and then executes a rotation about the Sun line to locate the Earth. The Sun/Earth acquisition is not always a favored approach because it can unnecessarily delay the return to normal earth-pointing operations.
The present invention overcomes problems in the art associated with lengthy earth reacquisition times. Thereby, the present invention can reduce downtime of a spacecraft and/or negative consequences of an orientation of a spacecraft other than a desired orientation.
A method for rapid earth reacquisition of a spacecraft. According to the method, a three-axis inertial attitude of the spacecraft is determined by rotating the spacecraft about its pitch axis while measuring star patterns. A pitch axis of the spacecraft is aligned with earth""s pole axis. The spacecraft is reoriented with respect to an earth-pointing reference frame.
The present invention also provides a system f or rapid earth reacquisition of a spacecraft. The system includes at least one star tracker operable to measure star patterns. At least one control actuator is operable to alter an attitude of the spacecraft. At least one processor is operable to determine a three-axis inertial attitude of the spacecraft by rotating the spacecraft about its pitch axis while analyzing star patterns measured with the at least one star tracker, aligning a pitch axis of the spacecraft with earth""s pole axis, and reorienting the spacecraft with respect to an earth-pointing reference frame by commanding the at least one control actuator to alter the attitude of the spacecraft.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from a review of the following detailed description. The detailed description shows and describes preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments and its several details are capable of modifications in various obvious respects, without departing from the present invention. Accordingly, the drawings and description are illustrative in nature and not restrictive.